1. Technical Field
This invention relates to printers for printing on a moving web and, more particularly, in a label printer printing on individual labels carried in spaced relationship on a surface of a longitudinally moving backing strip, to apparatus for adjustably sensing leading edge positions of the labels comprising, a stationary member carried above the backing strip on one side of the backing strip; a movable member carried by the stationary member for longitudinal movement along the path of the backing strip; adjusting means for adjusting a longitudinal position of the moving member; and, sensor means carried by the movable member for sensing leading edge positions of individual labels as a function of a change of thickness between the backing strip alone and the backing strip with a label on the surface thereof.
2. Background Art
In a label printer such as that generally indicated as 10 in FIG. 1, a plurality of labels 12 are releasably attached to a backing strip 14 forming a strip of media 15 that extends from a supply roll 16 over a plurality of guide rollers 18 to a printhead 20. At the printhead 20, ink from a ribbon 22 extending between a supply roll 24 and a take-up roll 26 is transferred to the labels 12. After printing, the labels 12 are separated from the backing strip 14 by a separator 27 and the backing strip 14 is wound onto a take-up roll 28 for later disposal. The labels 12 and backing strip 14 are moved in combination from the supply roll 16 to the printhead 20 by a driven platen roller 30 which also supports the labels 12 and backing strip 14 under the printhead 20 during the printing process. To keep the cost of the printer 10 low, the take-up roll 26, the take-up roll 28, and the platen roller 30 are all driven directly or indirectly by a single stepping motor 32 as indicated by the dashed lines. The movement of the stepping motor 32 is under the control of logic 34.
In label printing as in many areas, simplicity and cost are major factors. Consumers want the print quality they require in the least expensive printer. This is particularly true in on-demand label printers. And, the introduction of small, narrow labels such as employed as labels for printed circuit boards printed in the "picket fence" mode have made the problem even more severe. Accurate linear placement of the printing on these small labels is critical--particularly if there is any pre-printed material on them. And, at the same time, the cost of the printer is to remain low. Usually, such factors are a trade-off. That is, accurate placement can be obtained in a highly-complicated, high-cost printer. The prior art does not provide a way of obtaining both.
The problem is best understood with primary reference to FIG. 2 in combination with FIG. 1. A sensor 33 is positioned to sense the leading edge 34 of the label 12 next in line to move under the printhead 20. The sensor 33 senses the leading edge 34 at position 37 as indicated by the dashed line so labeled. The printhead 20 prints on the label 12 at the dashed line labeled 39. Thus, the dashed line 39 is the linear registration point for printing on the labels 12. What is needed, therefore, is a way of accurately positioning the labels one-by-one in sequence properly registered at the linear registration point represented by the line 39. In a copending application entitled METHODS AND APPARATUS FOR COMPENSATING STEP DISTANCE IN A STEPPING MOTOR DRIVEN LABEL PRINTER by Jay Miazga et al., Ser. No. 08/522,738, filed Aug. 31, 1995 and assigned to the common assignee of this application, the teachings of which are incorporated herein by reference, the usual technique for accomplishing such linear positioning is described in detail along with improvements which allow for dynamic adjustability of the process. Basically, the platen roller 30 as the primary moving force on the media 15 is driven by a stepping motor (not shown). Based on the labels 12 being equally spaced on the backing strip 14, the logic driving the stepping motor "knows" how many steps, "N", it takes to move the leading edge 34 from the sensing point 37 to the registration point 39. Thus, once the sensor 33 senses the leading edge 34 and outputs a signal on line 35 to the logic, the logic need only step the stepping motor N steps and the leading edge 34 should be properly positioned with respect to the registration point.
In a series of high-cost, low-tolerance printers, the distance between positions 37 and 39 could be held to close tolerances so that in any one of the printers, that distance would be N steps. As stated earlier, however, the demand in the industry among users is for label printers and the like which have both low cost and high performance capability. Low cost of manufacture on a large scale of necessity requires that tolerances be kept lower than possible in short-run, specialty items wherein the buyers are willing to pay the higher costs required. As a consequence, the distance between positions 37 and 39 will vary from printer to printer such that if the printing logic assumes N steps in each case, some printers will be in registration and some won't.
In addition, in another co-pending application entitled METHOD AND APPARATUS FOR ADJUSTING LATERAL IMAGE REGISTRATION IN A MOVING WEB PRINTER by the inventor herein, Ser. No. 08/522,033, filed Aug. 31, 1995 and assigned to the common assignee of this application, the teachings of which are incorporated herein by reference, a sensor is employed in one embodiment for sensing side edges of the media 15 for lateral positional adjusting.
Wherefore, it is an object of the present invention to provide methods and apparatus for making a low-cost web printer in which the linear positioning of labels and the like under the printhead is repeatable from printer to printer.
It is another object of the present invention to provide methods and apparatus for making a low-cost label printer in which the linear positioning of labels under the printhead is repeatable from printer to printer without regard to differences in path length caused by printer production tolerances.
It is still another object of the present invention to provide a sensor which is adjustable with respect to a linear sensing position.
It is yet another object of the present invention to provide a sensor which is adjustable with respect to both a linear sensing position and a lateral sensing position.
Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.